Unsharp masking is a well-known technique for enhancing the sharpness of processed images. Specifically, an original digitized image is stored from which a blurred version may be created by convolving it with a low pass filter (such as an unsharp mask). The difference between the blurred image and its original is multiplied by a factor K, and the resulting product is added to the original image to generate a sharpened image. The problem is how to compute the factor K by which the difference image is multiplied.
U.S. Pat. No. 4,571,635 to Mahmoodi et al. teaches that the factor K is computed in accordance with a formula which depends upon the standard deviation of the image pixels in a local neighborhood. While this technique results in some improvement in the sharpness of the image, it would be desirable to obtain further improvement in image sharpness without proportionately amplifying the noise in uniform areas of the image.